They look like the same colour but the walls are getting more light from the windows, also there is some smudging from the de-noiser you need more samples to get it clean.
Well… what do you think anyone will see in your image ? A wall and a celling which may be have some different lighting… and different dirt…
So if this is a CGI then you you added a lot of “realism” to it and as in the real world ceilings and wall might look different because of the windows (back lighting/ contrast)/
So what about your UV mapping, material setup, anything ??
(And now @Norman replied already 
)
I understand that, but my clients are very demanding and insist that it’s not the same color. Last time, I had to make different materials to emulate the same color so that they would be satisfied. So this time, I was wondering if I could do something different, like changing some specifications that I’m missing or something like that. 
Thank you. This is just a printscreen of the viewport in rendered mode, which is why there are some smudging.
They are right (in a sense), unless you use totally flat lighting, all the light sources the same colour, turn off all reflection, global illumination etc colours will always shift. But that would look totally unrealistic so it is a bit of a catch 22.
Unfortunately I think you will have to fine tune again until they are satisfied. 
The customer is not always right, but unfortunately they pay so 
Hello,
There, it’s me who doesn’t understand … 
Sometimes there are people like that. 
I don’t think there’s a solution in the rendering settings.
You will have to correct the color of the problem using Cryptomatte in Composing.
This can be quite awkward. Show the difference between the two outcomes.
He may recognize the weirdness or he may be satisfied.
…you may show them something like:
I find the IDENTICAL COLORS part (with A and B ) somekind of interesting.
Yes maybe Kim’s idea with compositor and Cryptomatte, totally de-saturate the walls and ceilings material, then tint the hue.
That should get rid of the hue shift due to global illumination (in theory).
